The Open Systems Interconnection (OSI) model partitions communication systems into abstraction layers. A given layer serves layers above, and is served by layers below. For example, a first layer serves a second layer, and the second layer serves a third. The first layer is a physical layer that defines physical specifications for a data connection. Physical specifications include connector layouts, cable specifications, and hardware specifications, among others. The second layer is a data link layer that provides a reliable link between two directly connected nodes. For example, Ethernet is a layer-two protocol that utilizes the physical layer to provide an Ethernet link between two nodes. The third layer is a network layer that provides procedures and functionality to define a network over which data sequences, i.e., datagrams, are transmitted among various nodes in the network. For example, internet protocol (IP) is a layer-three protocol that provides many capabilities, including routing functionality and IP addresses, among others.
One capability introduced in layer-three networks is loop suppression. IP introduces a time-to-live (TTL) attribute in an IP header that encapsulates a given packet. The TTL attribute can be used as an indicator of a loop's existence in a network. The general idea is that a packet should be discarded, or dropped, by the network after a certain number of hops to prevent infinite unicast or multicast loops.